CHAPTER XI. 



THE URINE. 



It is usual to speak of the urine as a secretion, but this is 

 not strictly correct ; speaking broadly, we may say a secre- 

 tion is something which is formed in a part for the purpose 

 of being eventually utilized by the system. This does not 

 apply to the urine, the chief ingredients of which are not 

 even formed in the kidneys, but only separated by them : 

 moreover, the urine having once been formed is of no further 

 use to the body, and is excreted. An excretion, therefore, 

 is a something removed from the system as being no longer 

 required, and the retention of which would be harmful. 



The kidneys may be regarded as the niters of the body, 

 and one of the channels by which waste and poisonous pro- 

 ducts are removed or filtered off from the blood, by which 

 means the latter fluid is maintained in a healthy condition. 

 We have seen how both nourishment and waste materials 

 are poured into the circulation, and we have studied several 

 of the channels by which the latter are removed, viz., 

 by the lungs, skin, and intestinal canal : we have now to 

 examine the last excretory path, viz., the kidneys. 



We regard the kidneys as the means by which the waste 

 products of the body are removed, or by which even 

 normal constituents of the blood are got rid of if in excess ; 

 and, according to Bunge, by the selective power which the 

 epithelial cells of the tubules possess, the alkalinity of the 

 blood is maintained, alkali being got rid of or sent back 

 to this fluid as it needs it. 



The structure of the kidney is peculiar, inasmuch as that 



